While they are both great mods, comparing them is like comparing Apples to Oranges. The VTR is slightly more advanced, at least in my opinion. It is heavier, requires the adapter for larger tanks or EGO connections, and has a removable battery. The MVP on the other hand is lighter, has a built in battery, and does not require and adapter for most tanks.
The decision would be up to you depending on what you are looking to get out of the mod.
While I like the MVP, I consider the built in battery to be a disadvantage as you need to charge the Mod, where the VTR can be used while a battery is charging. Other than that they both perform excellent.
To answer your adapter question: I have not seen any voltage loss using the adapter. Tanks vape just as well on the adapter as they do direct to the 510 connection. There are also a fare amount of tanks that do fit without the adapter.
Smoktech Trophy and Trophy 2
SmokTech PBC 2.5ml
Smoktech Pyrex U-DCT 2.5ml
Hope this helps.